Cobalamin cofactor synthase

As illustrated in Fig. 1, methionine synthase is positioned at the intersection between transsulfuration and methylation pathways. As a consequence, its level of activity exerts control over cellular redox status, since it determines the proportion of HCY that will be diverted toward cysteine and GSH synthesis. Methionine synthase activity is exceptionally sensitive to inhibition during oxidative stress, primarily because its cobalamin cofactor is easily oxidized (Liptak and Brunold, 2006). This allows methionine synthase to serve as a redox sensor, lowering its activity whenever the level of oxidation increases, until increased GSH synthesis brings the system back into balance. Electrophilic compounds, such as oxygen-containing xenobiotic metabolites, also react with cobalamin, inactivating the enzyme and increasing diversion of HCY toward GSH synthesis (Watson et al., 2004). Thus, methionine synthase is a sensor of both redox and xenobiotic status. 

The primary reaction catalyzed by methionine synthase converts homocysteine (Hey) and methyltetrahydrofolate (CH3H4folate) to methionine and tetrahydrofolate (Figure 2). Occasional oxidation of the reactive cob(I)alamin intermediate produces an inactive cob(II)alamin enzyme, which is reactivated by a reductive methylation that uses S-adenosylmethionine (AdoMet) as the methyl donor and flavodoxin or a flavodoxin-like domain as an electron donor. Thus methionine synthase supports three distinct methyl transfer reactions each involving the cobalamin cofactor. 

The deficiencies of cystathionine )5-synthase (CBS), sulfite oxidase, and methylenetetrahydrofolate reductase (MTHFR) may all result in central nervous system dysfunction, in particular mental retardation [1-3]. Defects of CBS and sulfite oxidase both cause dislocated lenses of the eyes, but the phenotypes are different otherwise. The manifestations of CBS deficiency, the most common of these disorders, and MTHFR deficiency range from severely affected to asymptomatic patients both may cause vascular occlusion. Deficiency of sulfite oxidase is clinically uniform, but genetically heterogeneous, and functional deficiency of the enzyme can result from several inherited defects of molybdenum cofactor biosynthesis [2, 4]. Hereditary folate malabsorption and defects of cobalamin transport (transcobala-min II deficiency) or cobalamin cofactor biosynthesis (cblC-G diseases) may cause megaloblastic anemia, in addition to CNS dysfunction [3, 5, 6]. 

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

Cobalamin (vitamin B12) Methionine cycle intermediate methyl carrier in the remethylation of homocysteine to methionine cofactor for methionine synthase... 

Methyl trap The sequestering of tetrahydrofolate as N -methyl THF because of decreased conversion of homocysteine to methionine as a result of a deficiency of methionine synthase or its cofactor, cobalamin (vitamin B ). 

Figure 3. Three of the modules comprising methionine synthase. At the top center is the Bi2- binding fragment [651-896], a structure with two domains, one a four-helix bundle that serves to cap the cofactor, and the other an a/p fold that interacts with the lower face of the corrin macrocycle and binds the nucleotide tail of cobalamin. Measurements of the rates of photolysis of the C0-CH3 bond indicate that the cap domain covers the upper face of the corrin in the substrate-free form of the intact enzyme (7). On the lower right is the activation domain [897-1227] with bound AdoMet. This helmet-shaped single domain is an unusual fold with no resemblance to other well-characterized AdoMet-binding domains (8). On the lower left is the structure of the methyltransferase AcsE from Moorella thermoaceticum, which we take as a surrogate for the folate-binding domain of MetH.

AdoCbl is found mainly in the mitochondria (Baneijee 1997). In humans, cobalamins serve as cofactors only for two enzymes the cytoplasmic methionine synthase and the mitochondrial methylmalonyl-CoA mutase. 

Figure 29.6 Pathways for the metabolism of homocysteine. Normal transsulfuration requires cystathionine P-synthase with vitamin Bg as cofactor. Reme-thylation requires 5,10-methylenetetrahydrofolate reductase and methionine synthase. The latter requires folate as cosubstrate and vitamin Bi2 (cobalamin) as cofactor. An alternative remethylation pathway also exists using the cobalamin independent betaine-homocysteine methyltransferase (Robinson 2000).

Vitamin B12 must be converted into its coenzyme forms, adenosylcobalamin and methylcobalamin, in the cell. These coenzymes function as cofactors of methylmalonyl-CoA mutase and methionine synthase, respectively. Chronic kidney disease (CKD) may affect the conversion from vitamin B12 to the coenzyme forms. This section describes the intracellular metabolism of cyanocobalamin, which is included in many dietary supplements, in particular, referring to a recently discovered trafficking chaperone called methylmalonic aciduria cdlC type with homocystinuria (MMACHC). Cyanocobalamin is first converted to cob(II)alamin, which has no cyanogen group on the ligand occupying the upper axial position of the cobalamin structure. Cob(II)alamin is further reduced to cob(I)alamin, which can function as a coenzyme in the body. 

Patients with methylmalonic aciduria and homocystinuria represent defective metabolism of cobalamin to the two cofactors methylcobalamin and deoxyadenosylcobalamin. Hence the activity of methionine synthase, and that of methylmalonyl-CoA mutase are defective [16]. The patients fall into two distinct complementation groups, designated cblC or cblD. An additional group of patients - designated group cblF - have defective transport of free cobalamin out of lysosomes [17]. 

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